This invention relates to a touch response sensor of a keyboard type electronic musical instrument to perform touch control upon depression of keys and more particularly, to an after-control sensor for the electronic musical instrument capable of varying the volume or the like of musical tones to be produced in response to intentional change of key depressing pressure after normal depression of the keys. Generally, the feelings of a performer can be expressed in musical tones through the key depressing operation of the organ by controlling the volume, pitch, tone-color or the like in response to the performer's finger touch in the attack and sustain states of the tones, which is called "touch control". More specifically, the volume, pitch, tone-color or the like of the tones being played may be changed responsive to a conscious variation of the key depressing pressure after a normal or initial depression of the keys to be played, which is called "after-control". For achieving such touch control there is needed a device for detecting the key depressing pressure upon depression of the keys, which device is required to have an insensible region with regard to the increase of the key depressing pressure. This insensible region means a range where the touch control is not effective under a normal depressing pressure so that the touch control is not executed in ordinary key depressing operation but is effected only upon a conscious increase of the key depressing pressure for performing the positive touch control. The device is also required to detect the key depressing pressure sensitively in response to the key touching operation by the performer's fingers once the touch control is conducted by the conscious increase in the key touching pressure.
Heretofore, as shown in FIG. 1, the prior art touch control sensor of the electronic musical instrument has an electrically conductive cylindrical rubber member 2 fixed to a predetermined position of the lower surface of a key 1, and stationary contact members 4 and 5 spaced in parallel with each other and fixed onto a base plate 3 and having contact pieces 4a and 5a, respectively of suitable configurations such as a substantial triangular shape and located at the position opposite to the rubber member 2. Upon depression of the key the conductive member 2 come into contact with the contact pieces 4a and 5a of the contact members 4 and 5 to electrically connect the contact pieces 4a with 5a. When the key depressing force is weak, the rubber member 2 is not so deformed in shape, as designated by a solid line in FIG. 2, and, accordingly, the area where the rubber member 2 is in contact with the contact pieces 4a and 5a is small. If the key depressing force is strong, the rubber member 2 is flattened as shown by two-dotted broken line in FIG. 2, and therefore the contacting area of the member 2 with the contact pieces 4a and 5a increases.
Thus, the contacting area of the rubber member 2 with the contact pieces 4a and 5a of the members 4 and 5 is varied in response to the strength of the key depression, resulting in variation of the resistance value therebetween in response to the key depression. If the contacting area increases, the resistance value decreases. The key depressing pressure is thus detected based on the variation of the resistance.
However, the aforementioned sensor is inconvenient for the after-control purpose because the cylindrical conductive rubber member is not distorted in exact proportion to the key depressing force when the key is further deeply depressed for the after-control since it becomes more easily deformable, if once distorted, due to its specific characteristics owing to its cylindrical shape. In addition, since the maximum contacting area of the rubber member of the after-control sensor is obtained in a state wherein the cylindrical rubber member is collapsed to almost half from its original configuration, the sensor cannot detect a stronger key depressing force. Another disadvantage exists in the complicated molding of the cylindrical conductive rubber members.